1. Field of the Invention
The present invention relates to a bearingless flowmeter, and more particularly to an improvement providing a single mode of rotation of the rotor element thereof.
2. Description of the Prior Art
A bearingless flowmeter is described in U.S. Pat. No. 3,447,373 to McNabb with improvements thereto shown in U.S. Pat. Nos. 3,927,568 and 4,015,474, to Hoppe.
This type of flowmeter utilizes a ring rotor disposed in a vortex chamber having a plurality of symmetrically spaced jets circumferentially around the chamber. A fluid flowing through the jets causes a rotation of the rotor with a rate of rotation proportional to the rate of flow of the fluid.
The bearingless flowmeter has been used in a kidney dialysis apparatus which extracts fluid from a patient's blood. In such apparatus, a dialysis solution is passed through a dialyzer separated from the blood by a semipermeable membrane. The pressure on the solution side is automatically controlled to cause water and low molecular weight solutes to pass from the blood through the membrane into the dialysate at a specified rate. A first flowmeter is disposed at the dialysate input of the dialyzer and measures the input flow rate thereof, which may be on the order of 500 ml per minute. A second flowmeter at the output measures the dialysate and removed fluid flow rate. The removed fluid typically produces an additional flow of one-tenth of the dialysate flow; for example, the total flow at the output of the dialyzer may be 550 ml per minute.
If the preselected difference in input and output rates is not achieved, the control system will change the differential pressure across the membrane to achieve such difference. If the flowmeters produce an erroneous reading, the control system will attempt to correct the apparent improper flow with possible serious consequences to the patient.
It is thus important that, after initial calibration performed before a dialysis treatment begins, the flowmeters remain stable. However, in dialysis apparatus using the above-described system, a serious problem has been noted. On occasions, certain discontinuities in the reading of a bearingless flowmeter occur during periods of constant flow. Two types of discontinuities have been noticed. The first is a step change and the second is a change having a finite transition time. Such changes always indicate a decrease in flow even though the flow rate is actually constant.
Various tests have been performed in an attempt to isolate the source of this problem. Not every flowmeter exhibited such discontinuities during testing. However, placing the ring rotor from a meter having the problem into another problem-free meter has shown that the problem follows the ring rotor. It is therefore theorized that slight imbalances in the ring rotor result in wobbling of the rotor when a resonance in the system occurs. Further, it appears that it is difficult to manufacture the ring rotor with sufficient uniformity to prevent slight imbalances from occurring.
Thus, there is a need for an improved bearingless flowmeter that will not exhibit the above-discussed problem.